1. Field of the Invention
The present invention relates to an apparatus for preventing sound distortion in hearing aids. More particularly, the present invention provides a hearing aid which utilizes gain control circuitry at both a preamplifier stage and an output stage to prevent saturation of input and output amplifiers.
2. Description of the Related Art
Distortion in the hearing aid art is defined as the generation of added undesired sounds in the output signal that are not present in the input signal. It is believed that such distortion, that is, these additional sounds, act as a type of masking noise for speech, both by direct masking of low-level speech cues that fill in the temporal structure of speech sounds, and by degrading important amplitude cues. Peak-clipping has been used as a technique for limiting the amplitude of the output stage of a standard hearing aid, however, the peak-clipping technique often creates distortions in the output stage of the hearing aid circuitry.
For the purposes of sound distortion analysis, a hearing aid can generally be considered to consist of two primary amplifying stages, a preamplifier stage and an output amplifier stage. FIG. 1 shows an exemplary configuration of a two stage hearing aid 10. As seen in FIG. 1, the preamplifier stage 12 of hearing aid 10 includes an amplifier 14 having one input 16 connected to a microphone 18 via coupling capacitor 20 and another input 22 connected to ground. The amplifier 14 has a fixed gain which is set by resistors 24 and 26. The output 28 of the preamplifier stage 12 is connected to a user adjustable volume control circuit 30 having an output 32 connected to the output stage 34 of the hearing aid 10. The volume control circuit 30 has a potentiometer 36 connected between the output 28 of the preamplifier stage 12 and an input 38 of the output stage 34 via coupling capacitor 40. The output stage 30 includes an amplifier 42 having one input 38 connected to the output 32 of the volume control circuit 30. The amplifier 42 has a fixed gain set by resistors 44 and 46 and has an output 48 connected to a transducer, for example, speaker 50.
In the preamplifier stage, the amount of undistorted amplification available is typically limited by the available battery supply voltage. In conventional ear-level hearing aids, the battery supply voltage is typically limited to 1.25 volts available from a zinc-air battery cell. If the input sound level (amplitude) increases dramatically, the resulting amplified signal at the output of the preamplifier stage tries to exceed the available battery voltage and thus the preamplifier saturates and the output signal becomes distorted (that is, it clips).
Various amplifiers, such as class A and class D amplifiers may be employed at the output stage. Such amplifiers are subject to an overload effect when the input sound level reaches certain thresholds. When using class A amplifiers in the output stage, a signal delivered to the output stage 34 from the preamplifier stage 12 increases as the amplitude of the input sound level into the preamplifier stage 12 increases. When the voltage at the output 28 of the preamplifier 12 reaches the limits of the battery supply voltage, no further amplification can take place. If the input sound level at microphone 18 continues to drive the preamplifier, the amplifier will saturate and distortion will occur.
A similar overload effect may also occur when using class D amplifiers in the output stage, though the saturation mechanism is different. Using a class D amplifier, the output stage 34 operates by producing a variable pulse-width modulated signal across the transducer 50, for example, the speaker coil. As either the input sound level or the amount of system amplification is increased, the pulses eventually merge into each other and the output signal reaches saturation, thus causing distortion, for example, in the form of peak-clipping. Typically, for a class D amplifier, distortion begins at about 3 dB below maximum acoustic output.
Peak-clipping may be unintentional, such as when the output amplifier is over driven, or peak-clipping may be intentional, such as when the saturated sound pressure level (SSPL) of a hearing aid is reduced by a peak-clipping circuit.
Saturation distortion that occurs due to over-driving or output clipping in an amplifier should be distinguished from low levels of distortion that can occur inside a hearing aid with low input levels. Saturation distortion occurs when the input level is so high that saturation is reached either in the pre-amplifier stage or in the output stage or in both stages. When this occurs, the waveform becomes highly distorted. Even in instances where a hearing aid is intentionally configured to produce low distortion at low input levels, saturation distortion will occur at some point as the input sound level is increased beyond the capability of the battery, the amplifier stage, and the output stage.
The high distortion effects of saturation can be seen graphically in FIG. 2 for a typical class D peak-clipping hearing aid used in the hearing aid circuit of FIG. 1. The peak acoustic gain of this hearing aid was 35 dB, the peak saturated sound pressure level (SSPL) was 107 dB, and the frequency response matrix slope (difference in acoustic gain between peak and 500 Hz) was 10 dB. As seen in FIG. 2 a plot of the harmonic distortion versus frequency for input sound levels of 60, 70, 80 and 90 dB is provided. These input sound levels correspond to soft speech, conversational speech, loud speech and shouted speech, respectively. The distortion performance graph shows that the level of distortion is low at low input sound levels, for example, 60 dB and 70 dB, and is in the area of 2 or 3 percent. When the input sound level is increased, for example, to 80 dB, the hearing aid goes rapidly into saturation and the level of distortion increases dramatically, with the percentage of distortion peaking at about 50 percent. At an input sound level of 90 dB, the percentage of distortion continues to increase and typically exceeds the 50 percent distortion plateau, especially in the mid-frequencies, for example between 900 Hz and 2000 Hz.
The use of a compression circuit at the preamplifier stage has been shown to slightly reduce the saturation distortion of conventional hearing aids. FIG. 3 illustrates conventional hearing aid circuitry with a compression circuit 52 connected to the preamplifier stage 12, and FIG. 4 is a distortion performance graph for the hearing aid shown in FIG. 3. As seen in FIG. 4, even with a compression circuit at the preamplifier stage, the percentage of distortion is still significantly high when the input sound level increases above the 80 dB level.
Therefore, a need exists for a hearing aid that responds to high as well as low input sound levels to substantially minimize the percentage of distortion in the output signal of the hearing aid so as to provide a person wearing the hearing aid with clear, audible sound.